Currently, and despite intensive development of fixed-culture processes, the majority of urban water treatment plants are equipped for the purpose of the use of the activated sludge process. In France, such a process provides for the treatment of approximately 80% of the total volume of discharged municipal effluents. The principle on which this process is based is that of the removal of carbon, nitrogen and phosphorus pollution from waste water using a complex culture of microorganisms, known as activated sludge. Waste water, after having been subjected to mechanical pretreatment stages, is brought into contact with the activated sludge in the presence of oxygen and the pollution is transferred from the liquid phase (waste water) to the solid phase (activated sludge). After the contact time necessary for the biochemical reactions which take place during the treatment, the mixture of water and sludge is conveyed to the clarifier, in order to be subjected therein to a final treatment stage: separation of the sludge and the treated water by settling. The settled sludge is then thickened in order subsequently to be recirculated to the head of the aeration tank, in order to take part therein in a new treatment cycle. This cycle is continually repeated. The pollution transferred into the solid phase is discharged in the form of excess sludge, which is extracted continuously.
An activated sludge is defined as being a mixture of different microbial species, such as bacteria, fixed or free protozoa, metazoa, fungi and algae. The microorganisms together form structured colonies in the form of flakes, which constitute the essential form for correct settling. The size and the nature of the flocs are determined by the operating conditions applied to the process.
The correct operation of the activated sludge process depends strictly on the operating conditions, such as the applied load, the age of the sludge, the hydraulic detention time, and others, applied to the system. In fact, these conditions have an influence on the selection of the microbial species of the activated sludge and consequently they determine the quality of it. The quality of the sludge and its composition influence, in their turn, the abilities of the sludge to settle. The stage which limits the activated sludge process is thus very often the separation by settling, this limitation resulting from the poor settling abilities of certain sludges. These problems are indicated by incidents due to filamentous bulking. In fact, under certain operating conditions, the activated sludge contains high amounts of microorganisms, known as filamentous microorganisms, which are characterized by a poor settling ability.
Moreover, when the activated sludge process has to provide for partial or complete removal of the nitrogen, the selection pressure resulting from the operating conditions has to provide for the presence both of species providing for the nitrification stage (autotrophic bacteria) and specifies responsible for denitrification (heterotrophic bacteria). Given the very different rates of growth of these two types of bacteria, it is not always obvious to apply optimum conditions in order to provide for the presence, in sufficient number, of the two populations. Again, it is noticed that the selection pressure and the microbial population dynamics play a decisive role in the process for the treatment of waste water by activated sludge.
With the aim of improving the operation and the performance of the activated sludge process, different techniques and variants, such as in particular hydraulic flow modifications, aeration system modifications or additions of different materials, have been proposed.
The modifications in the flow consist of the incorporation of selectors or of contact regions at the head of the aeration tank with the aim of creating the concentration gradient and of thus modifying the selection pressure exerted on the microbial species.
The additions of different materials, such as active charcoal (PAC), anthracite or diatomaceous earth, have the aim of improving, on the one hand, the removal of biologically resistant compounds or of bacterial inhibitors by their adsorbtion on the added material and, on the other hand, they contribute to a better formation of the flocs of activated sludge. These materials also act as support materials for bacterial growth on their surface. As a general rule, all the materials thus added are, after the treatment process, recovered and regenerated in order to be able to be reinjected into a new treatment cycle. Given their sometimes very high price, their constant loss in a significant amount cannot generally be envisaged, despite the costs related to their recovery and regeneration (in particular in the case of PAC).
Examples of the prior state of the art mentioned above will be found in particular in the following publications: U.S. Pat. No. 1,617,014 (addition of diatomaceous earth), FR-B-1,411,792 (addition of inorganic materials such as sand) or U.S. Pat. No. 2,059,286 (injection of PAC active charcoal).